A typical automotive-type solid electrolyte exhaust gas oxygen sensor is disclosed in U.S. Pat. No. 3,844,920 Burgett et al. It has a zirconia sensing element shaped as a tapered thimble. One thimble end is open and has a circumferential flange. The other end is closed and forms the most active part of the element. The interior and exterior of the thimble have separate porous electrode coatings of platinum or the like. The inner electrode is exposed to a source of oxygen, such as air or mixed metal oxide, for establishing a reference potential. The electrode has generally been formed by painting on a coating of platinum ink onto the zirconia thimble, drying the coating, and then firing the coated thimble at an elevated temperature. An improved technique by which it can be applied to the thimble is described and claimed in the copending U.S. patent application Ser. No. 080,449, entitled "Reference Electrode Process for Exhaust Gas Oxygen Sensor" that was filed Oct. 1, 1979 in the name of John Trevorrow and assigned to the assignee of this invention.
The outer electrode is exposed to the exhaust gas for establishing a potential determined by exhaust gas oxygen concentration. The outer electrode could be a porous thick film of platinum, like the inner electrode. However, it is preferred that this outer electrode be a thin film, applied by evaporation, sputtering, chemical vapor deposition or other such thin film deposition techniques. On the other hand, it has been difficult to consistently reproduce desirable properties, such as porosity and electrical parameters in the thin film electrodes. As a result, yields of satisfactory electrode properties have been limited, and various ancillary treatments have been developed to improve them. For example, U.S. Pat. No. 3,978,006 Topp et al discloses heating the solid electrolyte body after electrode deposition, to form pores in the electrode coating if it is not porous as deposited. U.S. Pat. No. 4,136,000 Davis et al discloses treating the electroded sensor element chemically and electrolytically to enhance sensor properties. U.S. patent application Ser. No. 89,264, entitled "Exhaust Electrode Process for Exhaust Gas Oxygen Sensor", filed on Oct. 29, 1979 in the names of Terry J. Gold et al and assigned to the assignee of this invention, discloses an improved sputtering process for producing porous platinum electrodes as deposited. Sensors having sputtered electrodes produced by the latter process consistently exhibit substantially stable response times of less than 600 milliseconds, particularly if heated once or twice in pure nitrogen at atmospheric pressure to 800.degree. C. for about 45 minutes before they are used. This nitrogen aging treatment is disclosed in U.S. patent application Ser. No. 030,748, entitled "Aging Treatment for Exhaust Gas Oxygen sensor", filed Apr. 17, 1979 in the names of Morris Berg et al and assigned to the assignee of this invention.
We have now found how to sputter platinum electrodes onto zirconia bodies in an even more improved way. We believe it provides improved consistency in electrode porosity and microstructure as deposited. In any event, it provides sensors that are fast and substantially stable as formed. Post-electroding treatments for activation and/or aging are unnecessary to obtain fast acting sensors. Moreover, rich-to-lean and lean-to-rich transition times are more balanced as formed and the sensors exhibit improved controllability. On the other hand, aging can still be beneficial, particularly in increasing the yield of fast acting sensors.